1. Field of the Invention
The present invention relates to a power converter protecting apparatus for protecting power converters connected in series or parallel to an electric power system for stabilization thereof against a system fault and/or a short-circuit fault.
2. Description of Related Art
For having better understanding of the invention, background techniques thereof will briefly be reviewed. FIG. 14 is a schematic circuit diagram showing a configuration of a conventional power converter connected in series to an electric power system which is disclosed in, for example, in "Transactions in The National Convention of Industrial Applications Section of The Institute of Electric Engineers of Japan", (1993), No. 103, p. 444. Referring to the figure, a self-excitation type SVC (hereinafter referred to as the power converters) is connected to the secondary winding of a series transformer 2 having a primary winding connected to a phase power line of an electric power system 1. The power converter 3 is implemented in the form of a three-phase bridge circuit comprised of gate turn-off switching elements such as thyristors (hereinafter referred to as the GTO switching elements) 3c1 to 3c6 to which diodes D are connected in anti-parallel, respectively, wherein a DC capacitor 6 for supplying a DC voltage is connected between the DC input lines with AC output lines being connected to the secondary winding of the series transformer 2.
Traditionally, the power converter 3 is connected in parallel to the electric power system 1 for the purpose of generation of reactive power and consumption thereof with a view to ensure stabilization for the electric power system. Besides, by connecting the power converters 3 in series to the electric power system 1, it is possible to change impedance of the electric power system 1. Thus, the power converters serve as very effective means for stabilizing the electric power system.
More specifically, the power converter serves for generating constantly an AC voltage orthogonal to a system current flowing through the electric power system. Thus, impedance of the electric power system can be changed in appearance by changing the amplitude of the AC voltage, which is functionally equivalent to a continuous control of a capacitor connected in series to the electric power system 1 and which can thus ensure very high effectiveness in realizing the stabilization of the electric power system. Besides, because the power converter 3 always generates the AC voltage orthogonal to the system current, necessity of providing a valid power supply source at the DC side of the power converter 3 can be eliminated.
In the conventional power converter apparatus known heretofore of the structures described above, the power converters connected in series to the electric power system are very effective for the stabilization of the electric power system. However, because of the serial connections of the power converters to the electric power system, an overvoltage may unwantedly make appearance in the electric power system upon occurrence of a fault in the electric power system or occurrence of a fault in the power converter apparatus or upon shut-down of the power converters and hence upon interruption of the system current. Thus, great difficulty is encountered in the protection and control of the power converter apparatus known heretofore.